The present invention relates generally to gas sensors, and, more particularly, to gas sensors including a light source and method of controlling light sources therefor.
The following information is provided to assist the reader to understand the invention disclosed below and the environment in which it will typically be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless clearly stated otherwise in this document. References set forth herein may facilitate understanding of the present invention or the background of the present invention. The disclosure of all references cited herein are incorporated by reference.
A number of gas sensors use energy from various types of light sources in the detection methodology of the gas sensor. For example, the use of gas sensors to detect the concentration level of gaseous species of interest using the photoacoustic effect is well known. For example, U.S. Pat. No. 4,740,086, the disclosure of which is incorporated herein by reference, discloses the use of a photoacoustic gas sensor to convert the optical energy of an amplitude modulated light source into acoustic energy when the light mechanically and thermally excites the gaseous species of interest as it diffuses into a sensing chamber upon which the light is incident. Sound waves of an intensity corresponding to the concentration level of the gas within the chamber are generated as the light radiation absorbed by the gas creates pressure fluctuations of a magnitude proportional to the number of gas molecules located within the sensing chamber. These sound/pressure waves are detected by a pressure sensor or acoustic detector such as a microphone.
Photoacoustic gas sensors can also have mechanical valves to let in the sample gas when open, which then close to trap the sample gas, block external acoustical noise and allow photoacoustic pressure to build up. Valves have the disadvantages of requiring energy to operate and of having moving parts which wear out leading to limited lifetimes (typical 0.5 to 3 years). Alternatively, gas diffusion element(s) such as described in U.S. Pat. No. 4,740,086, can be used to simultaneously allow gas diffusion, allow photoacoustic pressure to build up and attenuate external acoustical noise.
Whether a photoacoustic sensor includes a valve system or one or more gas diffusion elements, the operation and control of infrared light sources used therein continues to pose problems. In that regard, infrared light sources from different manufacturers can have different internal resistances. Care must be taken to avoid overheating or damaging various light sources with excessive electrical energy. However, under known control methodologies, overheating may occur, resulting in a decrease in the operational life of the light source.
Further, an infrared light source may have a lower electrical resistance when cold (such as at power up) than when at normal operating temperature. The electrical resistance of the light source when cold may even be too low to allow the power supply circuit of the photoacoustic gas sensor to start the light source. Today, manufacturers must build power supplies for photoacoustic gas sensors overly robust to start the power supply circuit under all temperature conditions, thereby requiring larger and/or higher rated components at additional cost. Also, a typical installation may have multiple photoacoustic gas sensors wired to a single power supply transformer that today must be overly robust to be able to start all the sensors at power up.
Similar problems can occur in other types of gas sensors in which energy from a light source interacts with an analyte to create an output signal. For example, such problems can occur in gas sensors in which the measuring method is based on the principle of light absorption in the infrared region known as non-dispersive infrared absorption (NDIR). In such sensors, broadband infrared radiation produced by the light source passes through a chamber filled with gas (for example, methane or carbon dioxide). The gas absorbs radiation of a known wavelength and this absorption is a measure of the concentration of the gas. There is typically a narrow bandwidth optical filter at one or both ends of the chamber to remove all other wavelengths of light before it is measured with an infrared detector or detectors, such as a pyroelectric detector or a thermopile detector.
In the case of a Light Emitting Diode (LED) or laser diode light source, it may be desirable to modulate the light source at higher frequencies than is desired for a photoacoustic sensor or an NDIR sensor. LEDs and laser diodes are typically non-linear semiconductor devices since the light output does not always increase in proportion to the increase in the input electrical power. For example, at the low end of the operating range, if the input power is doubled, the light output may more than double. At the upper end of the operating range, the lifetime of the LED or laser diode may be reduced by thermal effects of the electrical current flowing in the semiconductor junction. Generally, this leads to each device having an input power operating point with optimum efficiency and lifetime. Similarly, LEDs and laser diodes are often built to operate at frequencies ranging from D.C. to hundreds of megahertz (MHz). Operating a non-linear device at a modulated frequency allows the higher non-linear optical outputs while the electrical current is flowing and allows the device to cool when the electrical current is not flowing, for a reduced average power level for a longer lifetime. Thus the individual details of each device construction will also tailor an optimum modulation frequency, input power and lifetime for the device. This optimum modulation frequency may be higher than the desired frequency for a photoacoustic sensor or for an NDIR sensor.
It thus remains desirable to develop improved gas sensors, devices for use in gas sensors and methods of control of light sources used in gas sensors.